Japanese Unexamined Patent Publication No. 2006-157863 (Patent Document 1) discloses an acoustic sensor as one example.
This acoustic sensor has a structure in which a vibrating electrode plate (movable electrode) and a counter electrode plate (fixed electrode) face each other with a micro gap (air gap) provided therebetween. Because the vibrating electrode plate is formed by a thin film having a thickness of about 1 μm, when the vibrating electrode plate receives a sound pressure, the vibrating electrode plate vibrates microscopically in response to vibration of the sound pressure. A gap between the vibrating electrode plate and the counter electrode plate changes when the vibrating electrode plate vibrates. Therefore, acoustic vibration is detected by detecting a change in electrostatic capacitance between the vibrating electrode plate and the counter electrode plate.
The acoustic sensor is produced by utilizing a micromachining (semiconductor microfabrication) technique, and thus is of high sensitivity with micro dimensions such that one side thereof is several millimeters in a planar view.
However, in such an acoustic sensor, as illustrated in FIG. 1, a vibrating electrode plate 12 is firmly fixed to counter electrode plate 13 during production or use thereof (hereinafter, a state or a phenomenon, in which part or a substantially whole of the vibrating electrode plate is firmly fixed to the counter electrode plate to eliminate the gap, is referred to as sticking). When the vibrating electrode plate 12 sticks to the counter electrode plate 13, an acoustic sensor 11 cannot detect acoustic vibration because the vibration of the vibrating electrode plate 12 is obstructed.
FIGS. 2(a) and 2(b) are schematic diagrams illustrating a cause of generation of the sticking in the acoustic sensor 11, and FIGS. 2(a) and 2(b) are enlarged views of a portion corresponding to the portion X of FIG. 1. Because the acoustic sensor 11 is produced by utilizing the micromachining technique, for example, water 14 invades between the vibrating electrode plate 12 and the counter electrode plate 13 in a cleaning process after etching. Even in use of the acoustic sensor 11, moisture sometimes remains between the vibrating electrode plate 12 and the counter electrode plate 13 or the acoustic sensor 11 gets wet.
On the other hand, because the acoustic sensor 11 has micro dimensions, the gap between the vibrating electrode plate 12 and the counter electrode plate 13 is only several micrometers. Further, in order to enhance the sensitivity of the acoustic sensor 11, the vibrating electrode plate 12 has a thin film thickness of about 1 μm, and thus a spring property of the vibrating electrode plate 12 is weakened.
Therefore, in the acoustic sensor 11, sometimes the sticking is generated through a two-stage process as described below. In the first stage, as illustrated in FIG. 2(a), when the water 14 invades between the vibrating electrode plate 12 and the counter electrode plate 13, the counter electrode plate 13 attracts the vibrating electrode plate 12 by a capillary force P1 or a surface tension of the water 14.
In the second stage, after evaporation of the water 14 between the vibrating electrode plate 12 and the counter electrode plate 13, the vibrating electrode plate 12 sticks to the counter electrode plate 13 and this state is retained. An intermolecular force, an intersurface force, and an electrostatic force, which act between a surface of the vibrating electrode plate 12 and a surface of the counter electrode plate 13, can be cited as an example of a force P2 that firmly fixes the vibrating electrode plate 12 to the counter electrode plate 13 to retain the vibrating electrode plate 12 even after the water 14 evaporates. As a result, the vibrating electrode plate 12 is retained while sticking to the counter electrode plate 13, thereby disabling the acoustic sensor 11.
In the above description, the vibrating electrode plate 12 sticks to the counter electrode plate 13 by the capillary force of invading water in the first stage. However, sometimes the vibrating electrode plate 12 sticks to the counter electrode plate 13 due to a liquid other than the water, or sometimes a large sound pressure is applied to the vibrating electrode plate and the vibrating electrode plate thus sticks to the counter electrode plate. Further alternatively, the vibrating electrode plate sometimes generates static electricity so as to stick to the counter electrode plate, thereby causing the process of the first stage. In the following description, it is assumed that the vibrating electrode plate sticks to the counter electrode plate due to the water.
In order to reduce the sticking, an elastic restoring force Q of the vibrating electrode plate 12 is increased to overcome the capillary force P1 of the water 14 in the first stage or the retention force P2 in the second stage so that the vibrating electrode plate 12 is restored to the original state. In order to increase the elastic restoring force Q of the vibrating electrode plate 12, the film thickness of the vibrating electrode plate 12 may be increased to enhance the spring property. However, when the elastic restoring force Q of the vibrating electrode plate 12 is increased, the vibrating electrode plate 12 becomes hard to vibrate, which results in a problem in that the sensitivity of the acoustic sensor 11 degrades.
Alternatively, in the first stage, the sticking can be reduced in a case where the capillary force P1 is smaller than the elastic restoring force Q of the vibrating electrode plate 12. The capillary force P1 is increased with decreasing the gap between the vibrating electrode plate 12 and the counter electrode plate 13. Therefore, the gap is widened to decrease the capillary force P1. However, when the gap between the vibrating electrode plate 12 and the counter electrode plate 13 is widened, the thickness of the acoustic sensor 11 is increased to obstruct miniaturization of the acoustic sensor 11. The sensitivity of the acoustic sensor 11 also degrades.
Therefore, as illustrated in FIG. 3, in the acoustic sensor disclosed in Patent Document 1, the sticking between the vibrating electrode plate 12 and the counter electrode plate 13 is reduced by providing many projections 15 on the surface of the counter electrode plate 13 which faces the vibrating electrode plate 12. The projections are generally disposed at equal intervals on the entire counter electrode plate. It is well known that the retention force P2 between the vibrating electrode plate 12 and the counter electrode plate 13 has a correlation with a contact area between the electrode plates 12 and 13, and the retention force P2 is decreased with decreasing the contact area therebetween. Accordingly, when the projections 15 thinned as much as possible are provided on the counter electrode plate 13, the contact area between the vibrating electrode plate 12 and the counter electrode plate 13 (projections 15) is reduced to weaken the retention force P2. Therefore, the sticking of the vibrating electrode plate 12 is hardly generated.
According to the description of Non-Patent Document 2, because a ratio of a surface area to a mass is increased in a micro structure, an intersurface force acting between member surfaces plays a crucial role, and in particular a micro element having a diaphragm does not work occasionally with the diaphragm and a counter substrate stick to each other by the intersurface force. Non-Patent Document 2 further describes that the sticking of a cantilever can be reduced by providing a projection (stopper) on the cantilever.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-157863    Non-Patent Document 1: Shigeki Tsuchiya and five other, “Measurement of Intersurface Force and Reduction of Intersurface Force in Micro Structure”, Collection of Papers of the Society of Instrument and Control Engineers, vol. 30, No. 2, pp. 136-142 (1994), The Society of Instrument and Control Engineers, Japan